The invention relates to a method and an apparatus for locating defects in an electrical circuit with a light beam wherein the light beam disrupts a function of the electrical circuit at the scan point by releasing charge carriers.
In defect analysis of integrated circuits, a distinction must be made between causes of defects which are either founded in a defective design, or which can be attributed to technological deficiencies. Electron beam measuring technology works mainly in design optimization. A method and an apparatus for measuring potential at an electronic component with the assistance of electron beam measuring technology is disclosed, for example, in U.S. Pat. No. 4,277,679, incorporated herein by reference. Electron beam measuring technology is less suitable for locating technological defects than locating errors in the design of an electronic component.
So-called "Marginal Voltage Testing" (Microelectronics and Reliability, Vol. 22, No. 2, pages 241-264, 1982, Great Britain, incorporated herein by reference) is employed for defect locating in the inside of integrated circuits (see also British Patent No. 2,069,152). In this method for defect locating, the integrated circuit is driven with a high-frequency voltage wherein voltage pulses periodically repeat. Furthermore, the integrated circuit is scanned line-by-line with a focused laser beam. By releasing charge carriers in that region in which the laser beam impinges the integrated circuit, the laser beam disrupts the electrical function of the integrated circuit. When the laser beam impinges a location in the inside of the integrated circuit which has design and/or technology defects, then the disruption caused by the laser beam can lead to the failure of the logical function of the electrical circuit. The probability for the failure of the logical function of the electrical circuit increases with decreasing operating voltage with which the integrated circuit is operated. For this reason, in the prior art the operating voltage at the integrated circuit at every scan point impinged by the laser beam is lowered until the electrical circuit is non-functional. The lowest possible operating voltage at which the electrical circuit still functions even though the laser beam impinges a scan point within the integrated circuit is therefore a measure for the severity of the defect found at a scan point. The defect found at a scan point is all the more serious the higher the possible operating voltage at which the electrical function just still functions, whereby the electrical circuit becomes just non-functioning given a further lowering of this operating voltage.